Should I use C# or Python for AI?
If you're doing deep learning (which I assume you are, if you say you want to learn "AI"), then Python is a MUST. Virtually all the big frameworks are Python wrappers over a C++ core.

C# has no real deep learning frameworks. There are a couple such as the Microsoft Cognitive Toolkit, but they are on a completely different level from PyTorch or Tensorflow. No serious ML practitioner would do the majority of their research in a framework like that.

For more information, see: Why is Python the most popular language in the AI field?